For a tie in the cost of the index: at one time the choice was alphabetical by name but a fix came in some time in 10g to select the index with the larger number of distinct keys.

At present is seems to be:

If all indexes are unique and the costs are the same then tie-break on number of distinct keys, if those match then alphabetical.
If all indexes are non-unique and the costs are the same then tie-break on number of distinct keys, if those match then alphabetical.
If there is a mixture of unique and non-unique then NON-unique are preferred